Edgar Choueiri: BACCH(TM) 3D Sound

What It Is: BACCH(TM) 3D Sound is an audio technology that allows the listener to experience, through two ordinary speakers, truly three-dimensional (3D) sound with high tonal and spatial fidelity. The technology gives the listener of recorded sound the same 3D audio perspective as that of a listener located where the microphones were positioned in the original recording venue. For instance, if during a recording a fly circles the microphones, a listener of that recording played back through a BACCH™ 3D Sound system will hear the fly as if it were circling his or her own head. The technology is compatible with all commercial stereo recordings, which become 3D when played through the BACCH(TM) filter.

How It Works: In order to locate sound in 3D, the brain uses for cues the slight differences in the timing and volume between the sound reaching the left ear versus the right ear. Practically all stereo recordings (especially those made with dummy head microphones) contain such 3D cues, which are naturally coded as time and volume differences between the audio signals recorded on the left and right channels. If the audio signals recorded on the left and right channels of a stereo recording are transmitted to the left and right ears of a listener, respectively, the listener would hear in 3D. However, when a stereo recording is transmitted to the ears of a listener through loudspeakers, the sound on each channel is heard by both ears and therefore the perceived differences in timing and level are not the same as those that existed during the recording. In order for these differences to be perceived correctly by the brain of the listener, the sound on the left channel of the recording must not be heard by the right ear and that on the right channel must not be heard by the left hear. This unwanted transmission is called "crosstalk" and Choueiri's filter cancels the crosstalk allowing the listener to receive the proper 3D cues needed for the brain to locate sound in 3D.

Although crosstalk cancellation (XTC) technologies have been developed elsewhere, previous XTC filters cause noticeable changes to the tonal character of the sound. Choueiri's main contribution is to develop a mathematical technique for designing optimal crosstalk cancellation filters, called "BACCH(TM) 3D Sound filters," that add no coloration to the processed sounds. BACCH stands for ``Band-Assembled Crosstalk Cancellation Hierarchy," a name that represents the mathematical filter design method and pays tribute to Choueiri's favorite composer, Johann Sebastian Bach.

The BACCH(TM) 3D filters works by cancelling crosstalk without affecting the quality of the sound. Essentially, the filter forces the left speaker to emit, along with the sound intended for the left ear, an appropriately delayed “negative image” of the sound waves emitted by the right speaker, which arrives at the left ear just in time to mix with those waves and cancel them. The same is done for the opposite speaker.

The filter can do this because of the physical nature of sound. Sound consists of waves of air pressure traveling through the air. When two waves of the same height and wavelength travel in such a way that when one wave is cresting the other is in a trough, the two waves cancel each other out when they meet, and the result is no sound at all.

"As a result, the sound intended for the right ear reaches only the right ear, and the sound intended for the left ear reaches only the left ear," said Choueiri. "And the brain does the rest. This is exactly like what happens in 3D vision, when, for instance, you watch a 3D movie through special glasses that ensure that each eye sees only the picture intended for it."

Applications: BACCH(TM) 3D Sound is ideal for a wide range of applications including home audio, car audio, 3D Movies, 3D TV, computer gaming, teleconferencing, music recording, live broadcasting, security, military, and medical applications such as helping the hearing-impaired. The technology is far richer than Surround Sound, which simply surrounds the listener with sound from five or more speakers but does not create an accurate 3D experience.

Inspiration: Choueiri became an audiophile and amateur sound recordist at a young age. He has been recording musical ensembles, including the Princeton University Orchestra, as a hobby for years. In 2003, while in Europe to attend a conference in his field of spacecraft propulsion, he had the opportunity to walk into another conference where research problems with 3D audio were discussed.

Commercial Status: Over the past few months only, the university has been approached by several interested parties. Jawbone was granted a license and has produced a 3D audio portable Bluetooth speaker featuring the technology, which will be available worldwide February. Recently the university has signed an agreement with Cambridge Mechatronics Limited (UK), the world’s leading IP supplier of sound bars for televisions, to market BACCH(TM) 3D Sound with the company’s phased array technology to produce multiple sweet spots for 3D TVs (http://www.cambridgemechatronics.com/news/8March2011News). The goal is to have BACCH(TM) 3D Sound the standard on 3D TV sets.